Semiconductor device and electronic device

ABSTRACT

A channel forming region of a thin-film transistor is covered with an electrode and wiring line that extends from a source line. As a result, the channel forming region is prevented from being illuminated with light coming from above the thin-film transistor, whereby the characteristics of the thin-film transistor can be made stable.

BACKGROUND OF THE INVENTION

The present invention relates to the configuration of a thin-filmtransistor used in an electro-optical device such as a liquid crystaldisplay device.

In an electro-optical device that is represented by an active matrixtype liquid crystal display device, there is known a configuration inwhich thin-film transistors (TFTs) are used as drive elements andswitching elements. Thin-film transistors are configured using asemiconductor (usually, silicon semiconductor) thin film that is formedon a glass substrate by vapor-phase deposition. Thin-film transistorsare also used for an image sensor and other devices.

FIGS. 4(A) and 4(B) are a sectional view and a top view, respectively,of a one-pixel portion of an active matrix circuit using conventionalthin-film transistors. FIG. 4(A) is taken along line A-A′ in FIG. 4(B).A thin-film transistor whose cross-section is shown in FIG. 4(A) iscomposed of a glass substrate 401, an amorphous silicon or crystallinesilicon semiconductor active layer formed on the glass substrate 401 andhaving a source region 402, a channel forming region 403 and a drainregion 404, a gate insulating film 405 made of silicon dioxide orsilicon nitride, an interlayer film 407 made of silicon dioxide, a draincontact portion 412, a source contact portion 411, a drain electrode410, and a transparent conductive film (ITO film or the like) 408 thatis connected to the drain electrode 410 and constitutes a pixelelectrode. (The source and drain are reversed for a certain operation ofthe TFT.)

The source region 402 of the thin-film transistor shown in FIG. 4(A) isconnected to a source line 409 via the source contact portion 411. Agate electrode 406 is connected to a gate line 413. While usually thesource line 409 and the gate line 413 are perpendicular to each other,this is not always the case. In general, the gate electrode 406 and thegate line 413 are made of a metal such as aluminum or a semiconductorsuch as polysilicon added with phosphorus.

FIGS. 4(A) and 4(B) show a single pixel. Actually, an active matrixcircuit is formed by disposing at least one pixel as shown in FIGS. 4(A)and 4(B) at each intersection of the source lines and the gate lines. Aliquid crystal panel is configured by sealing a liquid crystal materialbetween an active matrix substrate on which the active matrix circuit isformed and an opposed substrate. There are following types of liquidcrystal display devices that use the liquid crystal panel having theabove configuration.

(1) Liquid crystal display is realized by applying light (back light) tothe liquid crystal panel.

(2) A video image is produced by applying high-intensity light to theliquid crystal display panel and projecting, onto a screen, light thatis transmitted from the liquid crystal panel. (Liquid crystal projector)

(3) A reflecting plate is disposed on the back side of the liquidcrystal display panel, and display is effected by causing external lightto be reflected by the reflecting plate.

In particular, where light is irradiated from the glass substrate sidein cases (1) and (2) above, it is necessary to shield the active layer,particularly the channel forming region, from the illumination light.This originates from the fact that the active layer (semiconductor layerdenoted by 402-404 in FIGS. 4(A) and 4(B)) is made of amorphous siliconor crystalline silicon such as polysilicon. In general, the resistanceof a silicon semiconductor varies when it receives light. In particular,when an amorphous silicon or crystalline silicon film is used, whichincludes dangling bonds, its electrical characteristics are greatlyvaried by illumination with high-intensity light. Further, theresistance of an intrinsic semiconductor (used for the channel formingregion) is varied more greatly by illumination with light than an N-typeor P-type semiconductor (used for the source and drain regions)Therefore, it is absolutely necessary to prevent the channel formingregion from being illuminated.

Where light 414 is incident from above the gate electrode 406 in thethin-film transistor (top gate TFT) having the structure in which thegate electrode 406 is located over the semiconductor active layer (seeFIG. 4(A)), it seems that no light may enters the channel forming region403 due to the gate electrode 406 serving as a mask.

Actually, however, part of the incident light goes around the gateelectrode 406, to enter the channel forming region 403. As a result, theconductivity of the channel forming region 403 is varied by theillumination and its characteristics are also varied.

That is, it is impossible for only the gate electrode 406 to completelyprevent light from entering the channel forming region 403. This problemis remarkable when the channel forming region and the gate electrodesare formed in a self-aligned manner. To solve this problem, a lightshielding layer or film is effectively used, but this increases thenumber of manufacturing steps.

SUMMARY OF THE INVENTION

An object of the present invention is to realize, without increasing thenumber of manufacturing steps, a structure of a thin-film transistorwhich is basically of the type shown in FIGS. 4(A) and 4(B) and in whichlight is not incident on nor enters the active layer, particularly thechannel forming region. In particular, the invention is effective whenapplied to an active matrix circuit having thin-film transistors whosesources and drains are formed by a self-align process that produces averge small overlap of the gate electrode and the source and drainregions.

The invention is characterized in that in an active matrix circuit, asource line or an electrode or wiring line (they cannot be clearlydiscriminated from each other) extending from the source line is soformed as to cover a channel forming region, to use the source line oran electrode or wiring line as a light shielding layer for the channelforming region. Since the channel forming region is included in aportion where a gate electrode and a semiconductor active layer overlapwith each other, forming a source line or an electrode or Wiring lineextending from the source line to cover such a portion is approximatelyequivalent to the above.

In the above structure, examples of a substrate having an insulativesurface are a glass substrate, a plastic substrate, a metal orsemiconductor substrate on whose surface an insulating film is formed.

FIG. 2(D) shows an example of the structure in which the source line orthe electrode or wiring line extending from the source line of thethin-film transistor is so formed as to cover at least the channelforming region. In FIG. 2(D), a source line electrode/wiring line 112that is connected to a source region 104 of a thin-film transistor via acontact portion 108 covers a channel forming region 105. That is, inFIG. 2(D), both of the electrode/wiring line 112 and the gate electrode107 serve as light shielding films for the underlying channel formingregion 105.

It may be possible to realize a similar structure by using anelectrode/wiring line for connecting the pixel electrode 103 and thedrain region 106. However, in such a case, a parasitic capacitancebetween the pixel electrode 103 and the gate electrode 107 becomes largeand a potential variation of the gate line 107 affects the pixelelectrode 107, causing serious problems in the operation of the activematrix. (For example, refer to H. Ono Kikuo et al: “Flat-Panel Display'91,” page 109.)

On the other hand, in the invention, a capacitive coupling between thesource line and the gate line due to a parasitic capacitance adverselyaffects the operation speed of the thin-film transistor, which does notinfluence the pixel potential. Therefore, no problem occurs in terms ofthe image display. Further, in the invention, a parasitic capacitancethat occurs due to the source line laid over the gate electrode does notcause any substantial problem, because it is smaller than 1/10 of theparasitic capacitance at the intersection of the source line and thegate line of the active matrix.

FIG. 1(A) is a specific top viewing of the structure whose cross-sectionis shown in FIG. 2(D). FIG. 1(B) is a circuit diagram corresponding toFIG. 1(A). As is apparent from FIG. 1(A), although the pixel electrode103 is connected to the drain region 106 via a contact portion 109 and awiring line, the wiring line does not cover the channel forming region.On the other hand, an extension 112 of the source line, which isconnected to the source region 104 of the thin-film transistor via acontact portion 108 is so formed as to cover the channel forming region(i.e., the portion where the gate electrode 107 and the semiconductoractive layer overlap with each other), and serves as a light shieldinglayer.

In summary, the problem that the characteristics of a thin-filmtransistor is varied or deteriorated due to illumination light comingfrom the electrode side can be solved by constructing theelectrode/wiring line that is connected to the source (or drain) of thethin-film transistor so that it serves as a light shielding layer forthe channel forming region. The electrode/wiring line that is connectedto the drain (or source) is connected to the pixel electrode.

To construct a liquid crystal display panel by using the invention, itis necessary that an active matrix substrate be illuminated from above,i.e., that a light source, an opposed substrate, and the active matrixsubstrate be arranged in this order. The light shielding effect of theinvention is entirely lost if the active matrix substrate is illuminatedfrom below, i.e., from the back side of the thin-film transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) and 1(B) are a schematic top view and a circuit diagram,respectively, of a one-pixel portion including a thin-film transistoraccording to a first embodiment of the present invention;

FIGS. 2(A)-2(D) show a manufacturing process of the thin-film transistoraccording to the first embodiment of the invention;

FIGS. 3(A)-3(D) show a manufacturing process of a thin-film transistoraccording to a second embodiment of the invention;

FIGS. 4(A) and 4(B) are a sectional view and a top view, respectively,of a conventional thin-film transistor for a one-pixel portion;

FIG. 5 shows a structure of a thin-film transistor according to a thirdembodiment of the invention; and

FIG. 6 shows a structure of a thin-film transistor according to a fourthembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIGS. 1(A) and 1(B) are a schematic top view and a circuit diagram,respectively, of a first embodiment of the present invention. That is,FIGS. 1(A) and 1(B) show a one-pixel portion of an active matrix typeliquid crystal display device. More specifically, FIG. 1(A) shows apixel electrode 103 provided for a pixel, a source region 104, a channelforming region 105 and a train region 106 together constituting aswitching thin-film transistor that is connected to the pixel electrode103, a source line 102 connected to the source region 104 via a contactportion 108, a gate electrode 107 formed on the channel region 105through a gate insulating film (not shown), and a gate line 101extending from the gate electrode 107.

FIG. 1(B) shows a thin-film transistor 110, and a liquid crystal 111that is connected to the drain region 106 of the thin-film transistor110 (actually, via a contact portion 109 as shown in FIG. 1(A)) andsupplied with a voltage from the pixel electrode 103.

FIG. 2(D) shows a cross-section taken along line B-B′ in FIG. 1(A). Inthis embodiment, as shown in FIGS. 1(A) and 2(D), the thin-filmtransistor is so configured that part of a source electrode wiring line112 covers more than half of the thin-film transistor. The source lines102 and the gate line 101 are formed of a metal such as aluminum, asemiconductor, or a laminated body thereof.

With the structure shown in FIGS. 1(A) and 2(D), since illuminationlight from the gate electrode side can be shielded by the extendingsource electrode wiring line 112, more than half of the thin-filmtransistor can be prevented from illumination. In particular, thechannel forming region 105 can be shielded from light that wouldotherwise come around the side faces of the gate electrode 107.

FIGS. 2(A)-2(D) illustrate a manufacturing process of the thin-filmtransistor shown in FIGS. 1(A) and 2(D). A description will behereinafter made of a manufacturing process of a thin-film transistorthat is provided for one pixel of a matrix. It goes without saying thatthe same structure is formed for each of a number of pixels arranged ina matrix.

First, a glass substrate 200 is prepared which may be any materialhaving an insulative surface. For example, it may be a semiconductor ormetal substrate on which surface an insulating film is formed.

A silicon dioxide film or a silicon nitride film as an undercoat film(not shown) is formed on the surface of the glass substrate 200, toprevent diffusion of impurities from the glass substrate 200 and toreduce stress during a heat treatment.

Then, a thin-film semiconductor 201 is formed which constitutes anactive layer of the thin-film transistor. In this embodiment, a1,000-Å-thick amorphous silicon thin film is formed by plasma CVD orlow-pressure thermal CVD (see FIG. 2(A)).

Since this embodiment needs a crystallized semiconductor layer as anactive layer, the amorphous silicon thin film 201 is crystallized inthis step. The crystallization is effected by a heat treatment (thermalannealing) of 600° C. for 12 hours in an inert atmosphere. To facilitatecrystallization, an element such as nickel may be added by a smallamount.

The crystallization may be effected by illumination with laser light orequivalent high-intensity light. Further, laser light or equivalenthigh-intensity light may be applied to a silicon film that has beencrystallized by thermal annealing. Where the thin-film transistor isallowed to have low-grade performance, the amorphous silicon film 201may be used as it is.

Then, the crystallized silicon semiconductor thin film is patterned intoa size of the intended active layer of the thin-film transistor. Thus,an active layer 202 of the thin-film transistor is formed. A1,000-Å-thick silicon dioxide film 203 to become a gate insulating filmis then formed by plasma CVD or sputtering.

Subsequently, a gate electrode 107 is formed by depositing a metal suchas aluminum or a polysilicon film doped with phosphorus and thenpatterning it, A gate line 101 (see FIG. 1(A)) is formed at the sametime.

Then, a source 104 and a drain 106 are formed by implanting phosphorusions: Since the gate electrode 107 serves as a mask, the phosphorus ionsare implanted into the regions 104 and 106, but not into the region 105.In this manner, the channel forming region 105 is formed at the sametime. Because of the implantation of phosphorus, the source 104 anddrain 106 are of an N type (see FIG. 2(C)). To obtain a P-type sourceand drain, boron may be implanted, for instance.

Next, annealing is performed by illumination with laser light to annealportions damaged by the above ion implantation and to activate implantedimpurity ions. In this embodiment, a KrF excimer laser is used. Thisstep may be effected by thermal annealing at a low temperature of about600° C. (see FIG. 2(C)).

Then, contact portions 108 and 109 are formed by forming a silicondioxide film 107 as an interlayer insulating layer, performingpatterning for hole formation, and forming metal wiring lines. As shownin FIG. 1(A), the wiring pattern is such that the source wiring line 112extending from the source line 102 covers more than half of thethin-film transistor.

It is particularly important that the source wiring line 112 be soprovided as to cover the top portion of the channel forming region 105.With this structure, illumination light from the gate electrode side canbe prevented from reaching the channel forming region 105, to therebyavoid a variation or deterioration of the characteristics of thethin-film transistor.

Embodiment 2

This embodiment relates to a thin-film transistor having a structure inwhich a gate electrode is made of aluminum, and an oxide layer is formedon the top and side faces of the gate electrode by anodic oxidation toincrease the breakdown voltage.

Referring to FIGS. 3(A)-3(D), a description will be made of amanufacturing process of a thin-film transistor according to thisembodiment. First, a 2,000-Å-thick silicon dioxide film as an undercoatfilm (not shown) is deposited on a glass substrate 300. An amorphoussilicon thin film 301 is formed thereon by plasma CVD or low-pressurethermal CVD (see FIG. 3(A)).

Then, the amorphous silicon film 301 is crystallized by subjecting it toa heat treatment of 600° C. and 12 hours, and then patterned into a sizeof the intended active layer of the thin-film transistor, to form anactive layer 302 of a crystalline silicon thin film. A 1,000-Å-thicksilicon dioxide film 303 serving as a gate insulating film is formedthereon by plasma CVD or sputtering (see FIG. 3(B)).

Next, a 5,000-Å-thick film mainly made of aluminum and serving as a gateelectrode is formed. By adding scandium of 0.1-0.5 wt %, for instance,0.2 wt % to aluminum, hillocks can be prevented from occurring in asubsequent anodic oxidation step etc.

Subsequently, the aluminum film is etched to form a gate electrode 304.An oxide layer 305 of 1,000-3,000 Å, for instance, about 2,000 Å inthickness is formed by performing anodic oxidation in an approximatelyneutral electrolyte using the gate electrode 304 as an anode. Thevoltage applied to the gate electrode 304 is about 120 V at the maximum.It is preferable that the oxide layer 305 have a sufficiently highbreakdown voltage. To this end, the oxide layer 305 is desired to besufficiently dense. The anodic oxide coating 305 produced in the abovemanner is called a barrier-type anodic oxide film, and has a breakdownvoltage that amounts to 90% of the maximum application voltage, which is120 V in this embodiment (see FIG. 3(C)).

Then, phosphorus ions are implanted into regions 306 and 307 to form asource 306 and a drain 307. A channel forming region 308 is formed atthe same time in a self-aligned manner. In contrast to the case of thefirst embodiment, this embodiment provides what is called an offsetstate in which the source 306 and drain 307 are spaced from the gateelectrode 304 by the thickness of the anodic oxide coating 305. Theoffset state is effective in reducing the leak current that ischaracteristic of the thin-film transistor.

Then, the ion-implanted regions are annealed and implanted ions areactivated by illumination with KrF excimer laser light (see FIG. 3(C)).

Subsequently, a silicon dioxide; film 309 of about 7,000 Å in thicknessis deposited as in interlayer insulating layer by plasma CVD. Theinterlayer insulating layer may be made of an organic resin or amultilayer of silicon dioxide and an organic resin.

Finally, a pixel electrode 310 is formed by ITO, and then etching isperformed for hole formation, and metal electrodes/wiring lines areformed. The metal electrode/wiring line 313 extends to the source lineand contacts with the source 306 via a contact portion 312. A drainelectrode/wiring line extending from a contact portion 311 is connectedto the pixel electrode 310.

Also in this embodiment, since the electrode 313 extending to the sourceline is so formed as to cover the source 306 and the channelforming-region 303 of the thin-film transistor, no light is irradiatedto the channel forming region 308. Thus, there can be realized thethin-film transistor that is free from a variation or deterioration ofthe characteristics due to light illumination.

In particular, where the oxide layer is provided around the gateelectrode that is mainly made of aluminum as in this embodiment, it iseffective to cover the channel forming region by the electrode/wiringline connected to the source. Since the aluminum oxide layer istransparent, if there were provided no shielding layer, light comingfrom the gate electrode side likely would reach the channel formingregion passing through the offset regions (parts of the channel formingregion 308 that are not covered with the gate electrode 304) or throughrefraction. By forming the electrode/wiring line 313 of this embodiment,the offset gate regions and the channel forming region can be preventedfrom being illuminated with light.

In the case of the first embodiment, there exists only the interlayerinsulating layer between the gate electrode and the source line.However, since the thin-film transistor portion is complicate instructure, i.e., has many steps, the single interlayer insulating layermay be insufficient in step coating ability, resulting in insufficientinsulation. In contrast, in this embodiment, because the anodic oxidelayer having a sufficiently high breakdown voltage is provided on thetop and side faces of the gate electrode in addition to the interlayerinsulating layer, the leak current between the source line and the gateelectrode can be reduced as much as possible.

In this embodiment, the N-channel TFT is formed because phosphorus isused as impurities. However, a P-channel TFT is generally preferable fora pixel of an active matrix to the N-channel TFT that suffers from alarge leak current. To form a P-channel TFT, boron may be doped insteadof phosphorus in the step of FIG. 3(C).

Embodiment 3

FIG. 5 shows a third embodiment of the invention. While the pixel andthe drain of the TFT are interconnected by the metal wiring line in thefirst and second embodiments, they may be interconnected directly by anITO film. In this embodiment, a layer including an ITO film and a layerincluding the source line are made different from each other. With thisstructure, electrolytic corrosion can be suppressed in patterning theITO film. Steps to the source line formation are approximately the sameas those in the second embodiment.

Referring to FIG. 5, a polysilicon active layer including a source 506,a channel forming region 508 and a drain 507 is formed on asubstrate/undercoat oxide film 500, and a silicon dioxide gateinsulating film 503 is formed thereon. The source 506 and the drain 507are of a P type. A gate electrode 504 is formed on the channel formingregion 508, and an anodic oxide film 505 is formed around the gateelectrode 504. An interlayer insulating layer 509 is formed so as tocover the TFT. First, a contact hole 512 for the source 506 is formed,and a source line 513 is formed there. Also in this embodiment, theelectrode and metal electrode/wiring line 513 that extends from thesource line covers the channel forming region 508.

Then, after a second interlayer insulating layer 514 is formed, acontact hole 511 for the drain 507 is formed. Silicon nitride, aluminumoxide and aluminum nitride, which are used as a passivation film, aremore appropriate as a material of the second interlayer insulating filmthan silicon dioxide. A pixel electrode 510 that is an ITO film isdirectly connected to the drain 507.

Embodiment 4

FIG. 6 shows a fourth embodiment of the invention. While in the first tothird embodiments, the channel forming region of the TFT is protectedfrom light coming from above, such structures may be combined with astructure for shielding the TFT from light coming from below (see FIG.6). This embodiment is characterized in that a light shielding film 614is provided under the TFT. The light shielding film 614 is made of ametal such as chromium and grounded. Referring to FIG. 6, the lightshielding film 614 is formed on a substrate 600, and a silicon dioxideundercoat film 602 is formed thereon. Further, a polysilicon activelayer including a source 606, a channel forming region 608 and a drain607, and a silicon dioxide gate insulating film 603 are formed.

A gate electrode 604 is formed on the channel forming region 608, and ananodic oxide film 605 is formed around the gate electrode 604. Aninterlayer insulating layer 609 is so formed as to cover the TFT, andcontact holes 611 and 612 are formed for the drain 607 And the source606, respectively. A metal electrode/wiring line 613 is connected to thesource 606, and a pixel electrode 610 that is an ITO film is connectedto the drain 607. Also in this embodiment, the metal electrode/wiringline 613 that extends from the source line covers the channel formingregion 608.

Although in FIG. 6, the source line and the pixel electrode are includedin the same layer, they may be included in different layers as in thecase of the third embodiment.

As described above, by forming the source line so that it covers thechannel forming region of the corresponding thin-film transistor, thechannel forming region can be prevented from being illuminated withlight coming from above the thin-film transistor. Therefore, a variationor deterioration of the characteristics of the thin-film transistor dueto light illumination can be avoided.

1. A semiconductor device comprising: a substrate; a semiconductor filmover the substrate; a source line connected to the semiconductor film; agate insulating film over the semiconductor film; a gate electrode overthe gate insulating film; an interlayer insulating film comprising anorganic resin over the gate electrode; a passivation film over the firstinterlayer insulating film; and a pixel electrode formed over thepassivation film, wherein the source line is overlapped with the gateelectrode.
 2. The semiconductor device according to claim 1, whereinsaid passivation film comprises any one of silicon nitride, aluminumoxide and aluminum nitride.
 3. The semiconductor device according toclaim 1, wherein said pixel electrode comprises ITO.
 4. Thesemiconductor device according to claim 1, wherein the semiconductordevice is a liquid crystal display.
 5. A semiconductor devicecomprising: a substrate; a semiconductor film over the substrate; asource line connected to the semiconductor film; a gate insulating filmover the semiconductor film; a gate electrode over the gate insulatingfilm; a first interlayer insulating film comprising an organic resinover the gate electrode; a second interlayer insulating film comprisingsilicon nitride over the first interlayer insulating film; and a pixelelectrode formed over the second interlayer insulating film, wherein thesource line is overlapped with the pate electrode.
 6. The semiconductordevice according to claim 5, wherein said pixel electrode comprises ITO.7. The semiconductor device according to claim 5, wherein thesemiconductor device is a liquid crystal display.
 8. A semiconductordevice comprising: a substrate; a thin film transistor comprising: asemiconductor film; a gate insulating film; and a gate electrode,wherein the gate insulating film is interposed between the semiconductorfilm and the gate electrode, a source line connected to thesemiconductor film; an interlayer insulating film comprising an organicresin over the thin film transistor; a passivation film over theinterlayer insulating film; and a pixel electrode formed over thepassivation film, wherein the source line is overlapped with the pateelectrode.
 9. The semiconductor device according to claim 8, whereinsaid passivation film comprises any one of silicon nitride, aluminumoxide and aluminum nitride.
 10. The semiconductor device according toclaim 8, wherein said pixel electrode comprises ITO.
 11. Thesemiconductor device according to claim 8, wherein the semiconductordevice is a liquid crystal display.
 12. A semiconductor devicecomprising: a substrate; a thin film transistor comprising: asemiconductor film; a gate insulating film; and a gate electrode,wherein the gate insulating film is interposed between the semiconductorfilm and the gate electrode, a source line connected to thesemiconductor film; a first interlayer insulating film comprising anorganic resin over the thin film transistor; a second interlayerinsulating film comprising silicon nitride over the first interlayerinsulating film; and a pixel electrode formed over the second interlayerinsulating film, wherein the source line is overlapped with the gateelectrode.
 13. The semiconductor device according to claim 12, whereinsaid pixel electrode comprises ITO.
 14. The semiconductor deviceaccording to claim 12, wherein the semiconductor device is a liquidcrystal display.